Tires/Gear Ratio/MPG - Fuel Economy, Hypermiling, EcoModding News and Forum

Dcarver · 09-04-2012, 07:43 PM

Hey y'all I've got a confusing (to me anyway) question. I'm looking to downsize my tires from 275/60/17 to probably 255/60/17. I figure this will effectively up my gear ratio from 3.55 to around 3.73ish. I know my lower gears are better for mpg and I love that, but I'd like to be able to tow better and with having 3.73 gears I would probably have better mpg while towing anyway. Now with smaller tires shouldn't I have less energy to rotate the tires effectively upping my mpg?

Please help me figure this out. I obviously don't want to change my gears as in mechanically swapping them and since I need new tires anyway, I just thought I could kill two birds with one stone.

thanks,
DC

royanddoreen · 09-04-2012, 07:47 PM

whats you're rpm at cruising speed now?

Dcarver · 09-04-2012, 07:52 PM

@ 70mph I'm just above 2000rpm. like maybe 2050 lol

CapriRacer · 09-05-2012, 11:33 AM

Quote:

Originally Posted by Dcarver

Hey y'all I've got a confusing (to me anyway) question. I'm looking to downsize my tires from 275/60/17 to probably 255/60/17. I figure this will effectively up my gear ratio from 3.55 to around 3.73ish. I know my lower gears are better for mpg and I love that, but I'd like to be able to tow better and with having 3.73 gears I would probably have better mpg while towing anyway. Now with smaller tires shouldn't I have less energy to rotate the tires effectively upping my mpg?

Please help me figure this out. I obviously don't want to change my gears as in mechanically swapping them and since I need new tires anyway, I just thought I could kill two birds with one stone.

thanks,
DC

I think you have that backwards. LARGER tires would reduce the effective ratio.

Think of it this way, a smaller diameter traveling at the same rpm generates a lower speed.

bobo333 · 09-05-2012, 12:08 PM

No he has it right, a higher number in the gear ratio = a higher engine speed vs road speed, smaller tires = higher gear ratio = higher engine speed vs road speed

RiderofBikes · 09-05-2012, 01:04 PM

Quote:

Originally Posted by Dcarver

I'm looking to downsize my tires from 275/60/17 to probably 255/60/17. I figure this will effectively up my gear ratio from 3.55 to around 3.73ish.

Shows your only changing the Width of the tire and not the Diameter. For towing capacity and safety, youd be better off keeping the stock width, of 275/ and just changing the Sidewall Hieght(changing ratio) /55R17 or /50R17. If thats what your are trying to get at?

Refer to Post #7 by MetroMPG
http://ecomodder.com/forum/showthrea...tml#post324301
Helpful RR chart on the second link, talks more about width of the tire.

oil pan 4 · 09-05-2012, 01:13 PM

You never want smaller tires on a tow vehicle.
You will be giving up a lot of weight capacity.

I see these are metric tires, unless they are LT truck tires you will need to derate their load capacity by 9% I think it is (I dont know why but the tire manufactures recomend this when putting a car tire on a truck or SUV, so do it).

The only reason you would ever want to consider a smaller tire when towing is if you were having trouble pulling the load.

Other than that bigger is better.

Taller Tires - EcoModder

CapriRacer · 09-05-2012, 03:40 PM

Quote:

Originally Posted by bobo333

No he has it right, a higher number in the gear ratio = a higher engine speed vs road speed, smaller tires = higher gear ratio = higher engine speed vs road speed

Opps! I got confusiated.

Quote:

Originally Posted by RiderofBikes

Shows your only changing the Width of the tire and not the Diameter........

No, the diameter changes as well.

Quote:

Originally Posted by RiderofBikes

......... For towing capacity and safety, you'd be better off keeping the stock width, of 275/ and just changing the Sidewall Hieght(changing ratio) /55R17 or /50R17. If thats what your are trying to get at?........

That would also change the diameter - but as is pointed out, this also reduces the load carrying capacity of the tire which is a terrible idea. Think Ford/Firestone and that's what we are talking about.

mcrews · 09-05-2012, 07:59 PM

the both have the same diameter according to tirerack.com 30" (275) 30.1 (255)
As someone else mentioned you have to change the second number

here ia tirerack chart for a tire that has both sizes
http://ecomodder.com/forum/showthrea...mpg-23182.html

royanddoreen · 09-05-2012, 09:01 PM

275/60/17's are 31.1" vs 255's 30". This should give you a 2115 rpm at 70.This is not a lot of difference but the load range goes down 50-70lbs on 2100 lbs per tire. Nothing to worry about unless you are going to use max loads in your truck.My guess is you will lose some mpg which could be restored by driving 65 mph.Hope this makes sense? Roy B